The uniformity of the coatings applied to manufactured objects and other substrates by various coating processes has been a long sought objective of the coating processes of the prior art. Various types of coating processes apply coating materials in ways which do not initially result in the desired uniformity of the thickness of the coating across the substrate. In many circumstances, the lack of desired uniformity is at least partially due to the shape of the object being coated. In electrostatic coating processes, for example, objects having corners, raised portions with short radii, or other projecting portions cause a concentration of the electrostatic field at such portions and this concentration causes a non-uniform deposition of the charged coating material. This results in a greater applied coating thickness at the more highly charged raised areas than is distributed by the electrostatic coating process elsewhere on the coated substrate. If allowed to solidify in its non-uniform state, the applied coating retains an irregular thickness. With most coating processes of the prior art, the irregular thickness of coating is tolerated and more than the ideal amount of coating material is applied to compensate for, and adequately cover, the more thinly coated regions.
The problem of non-uniform application of coating materials is not unique to electrostatic coating processes. In dip-coating processes, for example, the effect which substrate shape has on non-uniformities in the thickness of applied coatings is often even more of a problem than with electrostatic coating processes. The non-uniform thickness thus caused usually correlates with the characteristic contours of the object surface, tending to coat quite thinly at corners and raised areas of the substrate surface.
A serious problem is found to occur in applying coatings to running lengths of wire by dipping the wire into the coating material. In such applications, it is often the objective to apply an insulative coating to the wire. To insulate efficiently, the coating must achieve a minimum thickness everywhere across the surface. Excess coating at any point provides no advantage. The wire, if of a non-round cross-section, such as rectangular wire, has surface non-uniformities such as corners, ridges, or regions with short radii which will collect coating which is thinner in the regions of the corners of the wire, or in regions of shorter radii, than it is at the smoother or flat sides between the corners. The tendency of the flowable liquid dip coating material to thin out at the raised corners of the object is a common occurrence with dip coating processes. The non-uniform thickness of coatings so applied is usually attributed to the effect of the surface tension of the liquid coating. If allowed to solidify in the non-uniform condition, which most prior art processes cannot avoid, an inferior irregular thickness of the coating results. To prevent some regions from having a coating which is too thin, more coating than necessary is used to coat the flatter regions of the substrate surface so that an adequate minimum thickness is assured at the corners.
While various efforts have been made to control the deposition rate of coating material differently onto different regions of the object or substrate during application of the coating, the processes have been complex and expensive, and their results have at least partially been unsatisfactory. Objects nonetheless continue to be coated in the various commercial processes, such as the examples described above, with coatings which are not uniform particularly where substrate geometry which presents corners or other non-uniform surface curvature is involved.